Stop supporting EOL versions of Unity Editor

[webbertakken]

As brought up by @EliasVal:

Why are Unity versions that have reached EOL are still having tests ran with? 2018.x, 2019.x and 2020.x have reached their EOL's a long while back and I think that support should be dropped for versions that Unity doesn't support themselves

We should adapt the workflow to no longer build those outdated and end of life versions of the Unity Editor.